The invention relates to a hydraulic fluid screening element for inserting into a hydraulic line or into a hydraulic connection of a hydraulic device, in particular for inserting into hydraulic connections of hydraulic valves.
Because of their power density and their efficiencies, hydraulic systems or drives are today used in a wide variety of applications. In hydraulic systems, the delivery medium serves not only for power transmission. It also serves for generating and transmitting control signals. Used for this in particular are valves of many different types. It is necessary here to protect both the power-transmitting hydraulic circuits and the valves from contamination. Contamination leads to wear or even to damage and failure of the hydraulic systems, with corresponding consequential costs. The contamination of valves that are relevant to safety not only leads to them becoming worn, but also leads to risks for life and limb if it causes the control of the hydraulic systems to fail. Therefore, in all hydraulic systems there are powerful filters or filter systems that provide the required purity of the delivery medium. These filters are designed for filtering out extremely fine dirt particles from the delivery medium.
Fine filtering is a continuous process. When particles are introduced, for example as a result of wear, a specific purity class of the delivery medium must be maintained. The fine filters are arranged in hydraulic systems in such a way that they ensure an approximately constant good degree of purity of the delivery medium by continuous filtering. They consequently protect all of the elements of the hydraulic systems through which oil flows, including control valves, from dirt. The resultant level of contamination depends on the capability of the filter system, on the amount of particles produced within a period of time and the frequency of the circulation of the delivery medium.
Dirt in the widest sense comprises for example introduced particles that may originate from production residues but also from the wear of machine components. Particularly drastic effects are caused by the introduction of particles that may originate for example from coarse wear or the destruction of parts. These are generally particles of a kind that do not occur during normal operation but are so large that they can block assemblies such as valves, servo adjustment devices or simply just delivery-medium-carrying channels of a relatively small diameter. This type of contamination is an exceptional state that fine filters cannot quickly counter, because such fine filters are usually arranged at a central location and not in individual connecting lines or ahead of valves to protect them from malfunctioning or blocking if particles are introduced there.
For this reason, some valves relevant to safety are provided with separate filters, which inhibit or restrict the introduction of particles into particularly neuralgic functions. This type of filter does not serve for fine filtering but is intended to keep away particles that have a blocking effect. They are of course designed in terms of the filtering effect in such a way that less harmful fine particles are still allowed through and only particles that pose a risk are kept back. Because of the larger passages for allowing the delivery medium through, these filters are often also referred to as screens in the widest sense, as they are hereinafter in the description of the present invention.
There are hydraulic fluid screens of many different configurations. Very often, metal gauzes are used. There are single-layer and multi-layered screens. In the case of multi-layer screens, certain layers serve for improving the robustness and other layers serve for the actual function of providing protection from contamination. Other screens are elements that are provided with apertures, these being drilled, punched or introduced by means of laser or some other kind of machining. Water-jet cutting is also a suitable technology, but is not preferred because of the costs. There are also filters known as slot filters. In this case, structural elements are arranged in such a way that the inflow of delivery medium is only allowed through specifically remaining slots. A simple embodiment consists for example of a square profile that is arranged centrally in a bore in such a way that its four corners are supported on the wall of the bore and the side surfaces form slots by their distance from the wall of the bore. Other filters in turn consist of porous material, the filter element of which may be formed as a body, but also by a layering of fibres, spheres or the like that have a filtering effect.
All screening structures present a more or less viscosity-dependent hydraulic resistance. With increasing throughflow, the pressure drop across the screen increases. The higher pressure ahead of the screen puts load on the screen, and in the worst case can lead to its partial or even complete destruction. At the least, it then no longer has its screening function, which can be very difficult to detect in a hydraulic device and represents a concealed safety risk. There may, however, also be the release of already trapped particles, or the fragments of a destroyed screen may themselves become a high risk for the function of the hydraulic device.
Therefore, screening structures have to meet a series of requirements that are difficult to reconcile. Screens should be robust with respect to loads caused by the pressure of the delivery medium; they should be chemically resistant; they should have small passages of the same cross section; in spite of the small passages, their hydraulic resistance should be as low as possible and depend only little on the viscosity of the delivery medium; they must not under any circumstances contribute to contamination themselves; the structures should of course be robust, take up little overall space, be able to be produced inexpensively and be easy to install. In addition, they should be easy to clean and/or easy to exchange.
Screening structures of metal gauze are often not sufficiently robust and themselves represent a risk of contamination if they are damaged or even destroyed. Metal gauzes for screens/filters are cut out from large-area webs of gauze, the usually circular blank that is cut out giving rise to very short metal wires, which under loading become detached from the gauze structure and thus represent a risk for the functional capability of the hydraulic device.
Slot filters of the type described above require laborious processing in their production but also of the installation space. They require a large overall space and are expensive, but very robust. Punched or drilled elements often cannot be produced very finely. If the passages are produced by means of a laser, the associated introduction of heat reduces the robustness of the filters/screens. Lasered flat components are not very robust, but because of their thin-walled form have a low dependence of the hydraulic resistance on the viscosity of the delivery medium. In the case of thick-walled structures, the throughflow becomes laminar and the hydraulic resistance becomes great when there is a high viscosity of the delivery medium. Filters of porous material have uneven passages and are themselves a contamination risk on account of the risk of particles becoming detached.
The object of the invention is therefore to provide a hydraulic fluid screening element that is very robust, of a small form and chemically resistant, and can also be used under very high pressures. It should also be able to be produced inexpensively and be easy to install, and also allow itself to be easily cleaned.